Implantable leads are used in conjunction with a wide variety of medical devices to form medical systems for delivering therapy to a patient or sensing physiological parameters of the patient. For example, implantable leads are commonly connected to implantable pacemakers, defibrillators, cardioverters, or the like, to form an implantable cardiac system that provides electrical stimulation to the heart or sensing of electrical activity of the heart. The electrical stimulation pulses can be delivered to the heart and the sensed electrical signals can be sensed by electrodes disposed on the leads, e.g., typically near distal ends of the leads. In that case, the leads may be implanted such that the electrodes are positioned with respect to various cardiac locations so that the cardiac device can deliver pulses to or sense activity of the appropriate locations. Implantable leads are also used for stimulation of other tissue, muscle, or nerve, such as in neurological devices, muscular stimulation therapy, gastric system stimulators and other implantable medical devices (IMDs).
Occasionally, patients that have implantable leads may benefit, or even require, various medical imaging procedures to obtain images of internal structures of the patient. One common medical imaging procedure is magnetic resonance imaging (MRI). MRI procedures may generate higher resolution and/or better contrast images (particularly of soft tissues) than other medical imaging techniques. MRI procedures also generate these images without delivering ionizing radiation to the body of the patient, and, as a result, MRI procedures may be repeated without exposing the patient to such radiation.
During an MRI procedure, the patient or a particular part of the patient's body is positioned within an MRI device. The MRI device generates a variety of magnetic and electromagnetic fields to obtain the images of the patient, including a static magnetic field, gradient magnetic fields, and radio frequency (RF) fields. The static MRI field may be generated by a primary magnet within the MRI device and may be present prior to initiation of the MRI procedure. The gradient magnetic fields may be generated by electromagnets of the MRI device and may be present during the MRI procedure. The RF magnetic field may be generated by transmitting/receiving coils of the MRI device and may be present during the MRI procedure. If the patient undergoing the MRI procedure has an implantable medical system, the various fields produced by the MRI device may have undesirable effects on the medical leads or the medical device to which the leads are coupled. For example, the gradient magnetic fields or the RF fields generated during the MRI procedure may induce energy on the implantable leads (e.g., in the form of a current), which may be conducted to tissue proximate to the electrode and cause a rise in temperature of the tissue.